An active-matrix twisted-nematic liquid-crystal-display device (hereinafter abbreviated as "TN-LCD"), which uses a TFT element or an MIM element, exhibits excellent image quality comparable to that of CRT when viewed from the front face while holding inherent features such as a thin type, light weight and low consumption of electricity. Because of these features, a TN-LCD is widely used as a display device for note-type personal computers, portable televisions and portable information terminals. However, since a conventional TN-LCD is inherently associated with a problem of angle of visual field in that the display color varies or the display contrast decreases when the display is viewed obliquely because the liquid crystalline molecules have an anisotropy in refractive index. Accordingly, there is a strong demand for the solution of this problem, and various attempts have been made.
Methods already proposed and put to trial include, for example, a method wherein a pixel is divided into portions and applied voltages to these portions vary at a certain ratio (halftone gray scale method), a method wherein a pixel is divided into portions and the rising directions of liquid crystalline molecules in these portions vary (domain division method), a method wherein a lateral electric filed is applied to the liquid crystal (IPS method), a method wherein a vertically oriented liquid crystal is actuated (VA liquid crystal method), and a method wherein a bend-oriented cell and an optical compensator are combined (OCB method).
Although these methods are effective to some extent, they encounter eventual difficulty in production and cost-up, because these methods require to use different type of oriented films, electrodes, orientation made of liquid crystals and the like and because these changes require the establishment of production technologies and the installation of new production equipment.
Meanwhile, there is a method wherein an angle of visual field is widened by incorporating an optically compensating film into a conventional TN-LCD without changing the structure of the TN-LCD. This method, which requires neither change of the existing production facility of the TN-LCD nor additional equipment, is excellent in terms of costs and can be easily employed. Accordingly, this method is now drawing attentions and many proposals have been made in connection with this method.
The problem of the angle of visual field of a TN-LCD in a normally white (NW) mode is ascribed to the oriented state of the liquid crystal in a cell at the time that the voltage is applied and black is displayed. In this state, the liquid crystal is oriented in a nearly perpendicular direction and exhibits an optically positive uniaxiality. Therefore, the use of a film which exhibits an optically negative uniaxiality as an optically compensating film for the purpose of widening the angle of visual field is proposed in order to compensate the positive uniaxiality at the state of displaying black of a liquid crystal cell. It is also proposed to further widen the angle of visual field by use of a film of an optically negative uniaxiality having a tilted optical axis for the purpose of compensation, because the liquid crystal molecules in a region near to the boundary of the orienting film in a cell are oriented parallel to or tilted against the plane of the cell even when the crystal cell is in a black display.
For example, Japanese Patent Application Laid-Open (JP-A) Nos. 4-349,424 and 6-250,166 propose an optically compensating film using a cholesteric film having a tilted axis of helix and an LCD using this optically compensating film. However, it is difficult to produce a cholesteric film having a tilted axis of helix, and in fact the above-mentioned documents do not describe any method for tilting the axis of helix.
JP-A Nos. 5-249,547 and 6-331,979 propose an LCD using a negative uniaxial compensator having a tilted optical axis, and an concrete example of embodiment disclosed therein uses a multilayer thin film compensator.
JP-A Nos. 7-146,409 and 8-5,837 propose an optically compensating film prepared by the tilt-orientation of a discotic liquid crystal as a negative uniaxial compensator having a tilted optical axis and an LCD using this optically compensating film. However, the optical structure of the discotic liquid crystal is complicated and therefore the synthesis thereof is complicated. In addition, since the discotic liquid crystal is a liquid crystal having a low molecular weight, the formation of films of this liquid crystal requires a complicated process including a photo-induced crosslinking step. Accordingly, the industrial production encounters difficulties and eventual cost-up.
As another embodiment of the optically compensating film, also proposed is an oriented film using a liquid crystalline polymer having a positive uniaxiality.
For example, JP-A Nos. 7-140,326 proposes a compensator which is designed for use in LCD and which is composed of a twisted tilt-oriented liquid crystalline polymer and used for widening the angle of visual field of the LCD. This liquid crystalline polymer uses a liquid crystalline polymer compound (composition) composed of a bifunctional monomer. However, the procurement of the bifunctional monomer which is produced in an industrial scale is not easy, and it is not easy to induce a tilted orientation and a twisted orientation simultaneously in the industrial production of the compensator.
JP-A Nos. 7-198,942 and 7-181,324 propose a similar technique by utilizing a compensator for widening angle of visual field, which compensator comprises an oriented film of nematic liquid crystalline polymer, whose optical axis and the plane of the liquid crystalline film cross each other, and also an LCD using this compensator. This technique, however, does not sufficiently widen the angle of visual field, because this technique uses a compensator in which the optical axis is simply tilted.